Most phone devices today do not have both audio and text capability, but that will soon change. As there are more Internet Protocol (IP) devices available with text capability, users will grow accustomed to sending text as part of a voice conversation (e.g., Instant Messaging used while on the phone). When real-time text is integrated as part of a phone conversation between two end users, one of the end users may send text messages over the IP network to the other end user who is on the Public Switched Telephone Network (PSTN), an analog terminal adapter, and/or other device with traditional phones connected and is not capable of receiving and/or sending text. The text messages may be translated into audible tones which are played into the ear of the end user who does not have text capability much more frequently. For example, assume a PSTN caller calls another party to engage in a phone conversation, further assume that the called party is using a text-capable IP softphone. While talking, the IP softphone user says: “Here, let me send you the address”. He copies and pastes an address into the softphone device. In such a case, the text message may result in unwanted audio tones generated and played out to the PSTN user on the other end.
The ITU-T approved Recommendation V.151, which defines how to reliably transport text generated by a PSTN textphone (TTY/TDD) device over IP networks. Signals received by a gateway are extracted from the media stream from the PSTN and converted into text characters (T.140) that are then inserted into the media path. The primary application for this capability is trunking PSTN textphone calls over IP networks. However, when IP voice devices (e.g., a softphone or IP phone) are engaged in a text call, there is no indicator to alert the user of the IP devices to the fact that the remote party may be connected to the PSTN and is not capable of sending/receiving text. In such a situation, the user of the IP text-enabled device (e.g., Cisco's Unified Client or IP phone) may enter text, resulting in unwanted audio tones generated and played out to the PSTN user.
Therefore, it is beneficial to provide various clues to alert users of IP voice devices that a remote user may be connected to the PSTN and is not capable of sending/receiving text, thus avoiding transmission by the gateway of unwanted TTY (e.g., Baudot) tones out onto the PSTN to the user.